The present invention relates to medical measuring systems. In particular, the invention concerns a system and method for the measurement of cardiac output.
For the measurement of cardiac output (CO), two methods are previously known. In the single-dose method, a dose of salt solution at a temperature differing from blood temperature is introduced into the patient""s vena cava or right ventricle. The salt solution alters the temperature of the blood flowing through the heart, producing a thermal impulse in the blood circulation. The thermal impulse is measured from an artery using e.g. a thermistor connected to a Swan-Ganz catheter. Cardiac output can be computed on the basis of the time of propagation of the thermal impulse.
In the other method, a thermal impulse is generated by producing heat pulses in the blood in a vein e.g. by means of a heating resistor introduced into the vein via a catheter. For continuous cardiac output (CCO) measurement, the blood temperature is changed continuously. The cardiac output can be computed by comparing the temperature change detected by a thermistor in an artery to the change in the heating power supplied to the heating resistor.
FIG. 1 presents a simplified cross-sectional view of the heart 1 and a catheter 3 as described above, which has been introduced via the vena cava 6 into the right ventricle 7 and further into the pulmonary trunk 4. The catheter comprises a heating resistor 2 placed in the vena cava 6 and a temperature sensor 5 placed in the pulmonary trunk 4.
The CCO devices currently used are bulky because they use conventional technical solutions in their power supply circuits. To achieve a 10% accuracy in measuring the cardiac output, a power impulse of an accuracy of 1% is required. To ensure the safety of the patient, the heater of the catheter is typically powered using an alternating current at about 100 kHz. Low frequencies or a direct-current component in the electric current supplied to the heart may confuse the operation of the nervous system, which is particularly hazardous. The maximum values of the power impulses are typically 10 . . . 15 W. Galvanic decoupling has to be provided between the potentials of the power supply and the patient.
As the heating resistor and the cable connected to it have some inductance, prior-art solutions use a sinusoidal wave to minimize the effect of harmonics generated as a result of variation in the phase shift introduced by the catheter or the cable.
The source used to obtain a 100-kHz sinusoidal thermal impulse is generally an isolated chopper producing a floating-power rectangular main wave. The main wave is filtered using a large passive LC low-pass filter to obtain a sinusoidal wave to be applied to the catheter of the patient. As the capacitances vary, the filtering requires accurate calibration. Since passive filtering is used and calibration can only be performed before the measurement, prior-art systems cannot provide complete certainty regarding possible parameters varying during the measurement.
Specification U.S. Pat. No. 5,636,638 discloses an electric power amplifier used in CCO measurement. In the solution presented in this specification, the power supplied to the heating resistor is measured on the non-isolated side on the basis of current and voltage values. Correspondingly, the conversion from a rectangular wave into a sinusoidal wave is also performed on the non-isolated side. On the isolated side, the state and calibration of the catheter are monitored using a specific comparator circuit, from which the data is passed into a microprocessor controlling the system. The comparator circuit comprises a calibration resistor, and the value obtained from this resistor is used as a calibration reference.
The object of the present invention is to eliminate the drawbacks referred to above or at least to significantly alleviate them. A specific object of the invention is to disclose a new type of distributed system and method for the measurement of cardiac output that will allow a better control of the power supplied to the heating resistor. In addition, the present invention improves the technical diagnosis of the system. Intelligent functions implemented on the isolated side improve the reliability of the system as the last monitoring point is as close to the patient as possible. Thus, patient safety is also improved.
The basic idea of the invention is a distributed system for the measurement of cardiac output in which intelligent components are disposed on the patient side of an isolation interface.
The invention concerns a system for the measurement of cardiac output which comprises a temperature sensor placed in the vena cava and a heating resistor placed in the pulmonary trunk. The essential point about the placement of the temperature sensor is that its position in the blood vascular system is after the heating resistor or salt solution with respect to the direction of flow of blood. Moreover, the system comprises an isolation interface serving to separate an isolated side and a non-isolated side according to the potential of the patient. The energy to be supplied into the heating resistor is produced using a power source placed on the non-isolated side. According to the invention, the elements for the measurement of the power supplied to the heating resistor are disposed on the isolated side. The measuring elements are preferably current and voltage measuring elements, by means of which the effective power and the reactive power supplied to the catheter can be separated from each other.
Depending on the application, the effective power can be computed on different sides of the isolation interface. In an embodiment, the means for computing the effective power supplied to the heating resistor are disposed on the non-isolated side; in another embodiment, on the isolated side.
In an embodiment of the invention, the power measuring elements comprise a voltage measuring element and a current measuring element which measure the instantaneous value of the relevant quantity. In an embodiment, the power measuring elements comprise an A/D converter whose sampling frequency exceeds twice the highest substantial harmonic frequency of the rectangular wave produced by the power source. In an embodiment, the power measuring elements comprise an A/D converter whose sampling frequency is based on sub-Nyquist sampling. An embodiment of the invention comprises a low-pass filter disposed on the isolated side for filtering the rectangular wave fed to the heating resistor. The filtered power signal need not be a perfectly sinusoidal wave. The essential point is that the fast transients of the rectangular wave are sufficiently retarded to allow the application of lower requirements regarding the operating rate of the A/D converter used in the system. The frequency spectrum of an ideal rectangular wave is infinite, so it is not directly applicable as such for an advantageous A/D converter having suitable properties in other respects.
In an embodiment, the system comprises means for producing a technical diagnosis of the system from the reactive power. From changes in the reactive power, it is possible to detect e.g. a damaged cable insulation. This allows the system diagnostics to take appropriate steps e.g. by interrupting the supply of power to the catheter.
In an embodiment, the isolated side comprises means for the measurement of the resistance of the heating resistor. The resistance of the heating resistor changes as a function of temperature, so it needs to be known to allow the actual heating power to be computed. In an embodiment, the isolated side is provided with means for converting the blood temperature data into a digital form. These means comprise e.g. a suitable A/D converter, and in another embodiment they additionally comprise means for converting said digital form e.g. into a form acceptable to a suitable bus protocol.
In a preferred embodiment, the system comprises a microcontroller disposed on the isolated side. In this case, the means described above belong to an embedded system in which some of the functions are implemented via software. In this context, xe2x80x98microcontrollerxe2x80x99 refers to a processor-controlled device capable of controlling an external system. The microcontroller may also consist of e.g. a suitable micro-processor, a programmable logic circuit or an application specific integrated circuit (ASIC).
Moreover, the invention concerns a system which comprises means for changing the temperature of the blood in the vena cava, a temperature sensor placed in an artery and an isolation interface serving to separate an isolated side and a non-isolated side according to patient potential. According to the invention, the isolated side is provided with means for converting the blood temperature data into a digital form. The system on the isolated side is independent of the system used to produce a change in blood temperature. It may be e.g. a separate module collecting blood temperature data. Using a separate management system, it is possible to combine information obtained from several different modules, allowing the cardiac output to be computed from the temperature data.
The invention further concerns a method for the measurement of cardiac output in a system as described above. In the method, the power supplied to the heating resistor is measured on the isolated side. In an embodiment, the effective power supplied to the heating resistor is computed on the non-isolated side; in another embodiment, on the isolated side. For the computation of the power, preferably the instantaneous value of the voltage and current is measured.
In an embodiment, the sampling frequency used for the power measurement exceeds twice the highest substantial harmonic frequency of the rectangular wave produced by the power source. In another embodiment, the sampling frequency used for the power measurement is based on sub-Nyquist sampling. The rectangular wave to be fed to the heating resistor is low-pass-filtered preferably on the isolated side. In an embodiment, a technical diagnosis of the system is produced using the reactive power value obtained in the power measurement. In an embodiment, the resistance of the heating resistor is measured on the isolated side. The resistance can be derived according to Ohm""s law from the voltage and current wave forms obtained in the power measurement.
The blood temperature data is preferably converted into a digital form on the isolated side. In an embodiment, a microcontroller is provided on the isolated side.
As compared with prior art, the advantages of the present invention include the fact that, by placing the active components on the isolated side, a cheaper implementation can be achieved. The power signal transmitted across the isolation interface need not be exactly sinusoidal to minimize the effects of unwanted harmonic components. The filtering can be performed using cheaper components. The present invention also makes it possible to transfer the measurement result in a digital form across the isolation interface. The advantage thus achieved is considerable; the signal level does not deteriorate in crossing the interface, and the technical operation required for the crossover is quite simple. In addition, the system improves patient safety in the form of an improved system diagnosis. As the current and voltage waveforms are monitored at a point just before the catheter, the control data needed for the regulation of the system as a whole is as accurate as possible.
In a solution according to the present invention, no separate calibration resistors are needed on the isolated side because the system receives the information directly from the isolated side. A distributed system is also flexible; different functions can be combined to form distinct functional units, modules that can be removed or added to the system as necessary.